Originally posted by regislea What would be the best AF set up for such a portrait, that gives me sharp focus on the eyes even thought the sitter may be mobile??
As Adam says, try shooting with Live View, making sure Face Detection is enabled (it should be on by default with the K-3).
If using the viewfinder, try settings like AF.C, focus priority selected, and then selecting a focus point close to the eye, whilst enabling 'Expanded Area AF (S)' to keep a tight track on the movement of the target AF point. [pp 42-46 of the K-3 manual].
For reference, the K-3'a AF point distribution looks like this:
Or just shoot as you would normally, but at f4 or f5.5 instead of f2.8. The extra DOF will solve your problem.
Also maintaining a tight focus on just one feature like the subjects eye, whilst shooting wide open, runs the risk of having the eye in focus, but the nose or lips out of focus. I've seen many portraits ruined by photographers giving priority to bokeh over focus depth when shooting portraits.